The joining structure for side members 1 and cross members 2 of a chassis frame according to an embodiment are shown in FIG. 1 and FIG. 2. The side members 1 are located opposite each other at a prescribed distance in the width direction of the vehicle. Cross members 2 are located to span the opposite first and second side members 1 (see FIG. 7). Preferably, the first and second side members are mirror images of each other. Improvements as explained below are made to the joining structure for side members 1 and cross members 2 to eliminate dangerous sites in terms of strength (areas of weakness) without loss of joined rigidity, and to suppress occurrence of cracking in welds due to inputs during running of the vehicle.
The cross member 2 is preferably a tubular member having a flange 4 that is formed integrally at both ends thereof via a flare 3. Flange 4 extends circumferentially about the end of the cross member. The flare 3 is formed by smoothly bending outwards the end of the cross member 2 through 90° in the radial direction, and the flange 4 is formed integrally with the flare 3 and extended to the vicinity of a bend 12 (explained later). As also shown in FIG. 4, cylindrical extension members 5 are inserted and press fitted into the ends of the cross member 2 provided with the flares 3 and the flanges 4 to the prescribed depth to project outwards from the flanges 4 lengthwise of the cross member and in the lateral direction of the vehicle. The extension member 5 is welded at the bend of the flare 3 around its periphery, thus forming the first weld bead 6 shown in FIG. 2 and FIG. 4.
As shown in FIG. 1 and FIG. 2, side member 1 has a closed cross-sectional part 8, preferably box shaped. The closed cross-section part 8 may be provided along the entire length of the side member 1 in the longitudinal direction, however, it need only be provided at the location wherein the cross member 2 is attached. The closed-section part 8 is formed in a rectangular cross-sectional shape comprising an inside part 8i in the lateral direction of the vehicle, an outside part 8o in the lateral direction of the vehicle, a top part 8u, and a bottom part 8d. Inside hole 9i and outside hole 9o where the afore-mentioned extension member 5 is inserted are formed in the inside part 8i in the lateral direction of the vehicle and the outside part 8o in the lateral direction of the vehicle respectively. The inside hole 9i is formed to a diameter greater than the outside diameter of the extension member 5, and the outside hole 9o is formed to a diameter matching the outside diameter of the extension member 5.
The afore-mentioned cross member assembly 7 is joined (welded) to the side member 1 as follows. The extension member 5 of the cross member assembly 7 is passed through inside hole 9i and outside hole 9o, until the flange 4 abuts against the inside part 8i. The cross member assembly 7 is placed in a jig to hold it in position in relation to the side member 1. Since inside hole 9i is larger than the outside diameter of extension member 5, ease of work is improved when passing through. A distal end of extension member 5 frictionally fits within outside hole 9o. In this condition, the outer periphery of the flange 4 is welded in the peripheral direction to the inside part 8i by second weld bead 10. A third weld bead 11 is formed by welding the extension member to the outside part 8o. 
According to the afore-mentioned joining structure, the first weld bead 6 is formed on the flare 3, in other words, on a part where the end of cross member 2 is bent and having high rigidity, and thus occurrence of deformation based on the afore-mentioned inputs is inhibited. Furthermore, since the first weld bead 6 is located on the inside of the cross member 2, stress occurring due to the afore-mentioned various inputs is less than with the first weld bead w1′ located on the outside of the cross member c in the type in FIG. 10 and FIG. 11. As a result, stress concentration of the first weld bead 6 is alleviated, and occurrence of cracking in the first weld bead 6 is suppressed. In other words, according to the present embodiment, parts equivalent to the first weld bead w1′ being a dangerous site in terms of strength (sites wherein the possibility of cracking due to stress concentration is high) in the type in FIG. 11, are eliminated.
Furthermore, in relation to the second weld bead 10, occurrence of cracking based on the afore-mentioned various inputs is suppressed for the same reason as the second weld bead w2′ in FIG. 10 and FIG. 11 explained previously. In other words, the top and bottom of the second weld bead 10 are located near the top and bottom bends 12 of side member 1, and since occurrence of deformation based on the afore-mentioned inputs is inhibited, occurrence of cracking in the second weld bead 10 can be suppressed by suppressing progression of stress concentration based on deformation of the parent material (side member 1) in the second weld bead 10, absorption of load by deformation of the flare 3, and by an ability to alleviate stress occurring with increased length of the second weld bead 10. Sufficient length of the top and bottom of this second weld bead 10 may be ensured near the bend 12 at top and bottom of side member 1 by forming the flange 2 in a rectangular shape having greater length in the top-bottom direction.
In relation to the third weld bead 11, the majority of the force transmitted from the cross member 2 to the side member 1 due to the afore-mentioned inputs is distributed by the second weld bead 10 being of sufficient length, and the stress on the third weld bead 11 is reduced, and thus occurrence of cracking in the third weld bead 11 is suppressed. While the second weld bead 10 directly joins the flange 4 formed integrally in the cross member 2 and side member 1, the third weld bead 11 indirectly joins cross member 2 and side member 1 via extension member 5, and thus the force transmitted from the cross member 2 to the side member 1 applies a greater load to the second weld bead 10 directly than is applied to the third weld bead 11 indirectly via the extension member 5. The second weld bead 10 is long (weld surface area is greater) and thus stress is reduced and cracking does not occur. On the other hand, the third weld bead 11 is short and the force applied therein is small, and thus the stress occurring is small and cracking does not occur. The plate thickness of extension member 5 is determined in accordance with the degree of rigidity required, however in many cases it is less than the plate thickness of the cross member 2, as shown in FIG. 2a and identified as extension member 5a. 
Thus, according to the joining structure for the side member 1 and the cross member 2 of the present embodiment, in relation to the first weld bead 6 and the second weld bead 10, the plate is welded near the flare 3 and the bend 12, respectively, close to the part where the plate is bent and having a rigidity greater than the flat part, and thus deformation due to the afore-mentioned various inputs is reduced and progression of stress concentration is suppressed, and occurrence of cracking is suppressed. In relation to the third weld bead 11, the majority of the force transmitted from the cross member 2 to the side member 1 is distributed over the second weld bead 10 having greater weld length, stress in the third weld bead 11 is alleviated, and occurrence of cracking in the third weld bead 11 is suppressed.
In relation to the joined strength of the side member 1 and the cross member 2, the first weld bead 6 is welded to the flare 3 where the plate is bent through 90°, and thus, as shown in FIG. 10 and FIG. 11, the contact area of the bead with the parent material increases to a greater extent than the first weld bead w1′ welded to the peripheral face of the cross member c, and joined strength increases. Since the second weld bead 10 is formed on the outer periphery of the flange 2, the welded area becomes greater than that of the first weld bead w1 shown in FIG. 8 and FIG. 9, and joined strength increases.
In other words, according to the joining structure for the side member 1 and the cross member 2 according to the present embodiment, dangerous sites in terms of strength (areas of weakness) can be eliminated without loss of joining rigidity, and thus occurrence of cracking in welds 6, 10, and 11 due to the inputs during running of the vehicle can be suppressed. As a result, the chassis frame adopted in the afore-mentioned joining structure can maintain the prescribed rigidity and strength with thinner plate thickness than the types in FIG. 8 through FIG. 11, and can provide chassis frames lighter in weight with lower cost.
In the modification shown in FIG. 5, the tip 5a of the afore-mentioned extension member 5 is gradually reduced in diameter towards the outside in the width direction of the vehicle. By reducing the diameter in this manner, ease of inserting extension member 5 into the afore-mentioned outside hole 9o is improved.
In the modification shown in FIG. 6, the cross member 2 is manufactured by assembling parts 2a and 2b having the shape of a pipe split in two. The afore-mentioned parts 2a and 2b are formed by a pressing process and the like.
Furthermore, the cross member 2 is not limited to a member of cylindrical shape as shown in the figure, and may be a member wherein the central part is of an open U-shape. Furthermore, the side member 1 is not limited to a pipe material as shown in the figure, and may be formed from two pieces of materials of U-shaped cross-section assembled one above the other or side by side.
Furthermore, the joining structure according to the afore-mentioned embodiments may be applied to either all of the cross members 2 or some of cross members 2 of the chassis frame.
The foregoing specific embodiments have been provided to illustrate the structural and functional principles of the present invention, and are not intended to be limiting. To the contrary, the present invention is intended to encompass all modifications, alterations, and substitutions within the scope of the appended claims.